Heretofore, it is well known that, in a fluid pressure circuit for operating a fluid pressure cylinder, a pilot check valve is provided in order to appropriately control the flow of a working fluid (for example, see Japanese Laid-Open Utility Model Publication No. 03-025080). For example, in a fluid pressure circuit in which the fluid pressure cylinder is used as a lift cylinder, a head-side pressure chamber of the cylinder and a rod-side pressure chamber thereof are connected to a pressure supply source through a switching valve, and a pilot check valve is arranged in a flow channel between the rod-side pressure chamber and the switching valve.
The pilot check valve is equipped with a poppet valve plug that is elastically urged toward a valve seat by a spring, and a pilot piston facing the poppet valve plug and which is disposed slidably in an axial direction thereof. The pilot piston presses the poppet valve plug when a pilot pressure is applied. When a supply pressure is applied to the head-side pressure chamber of the cylinder, the pilot pressure is applied to the pilot check valve.
In the fluid pressure circuit as above constructed, when the working fluid is supplied from the pressure supply source to the pilot check valve through the switching valve, under pressure based on the working fluid, the poppet valve plug is separated away from the valve seat against the elastic force of the spring, whereby the working fluid flows into the rod-side pressure chamber of the cylinder, to thereby press the piston of the cylinder upwardly. When the piston reaches an upper end position, the pressure difference between the upstream side and the downstream side of the poppet valve plug becomes zero, and as a result, the poppet valve plug is seated on the valve seat by the elastic force of the spring. Thus, even if supply of the pressure from the pressure supply source is stopped, the poppet valve plug remains closed, and therefore the cylinder is prevented from dropping down.
On the other hand, when the working fluid is supplied from the pressure supply source to the head-side pressure chamber of the cylinder through the switching valve, the pilot piston is advanced by the pilot pressure, and presses the poppet valve plug, whereby the poppet valve plug is separated away from the valve seat. Consequently, the fluid in the rod-side pressure chamber of the cylinder is discharged from the switching valve through the pilot check valve, so that the cylinder is lowered.
In the above fluid pressure circuit to which the conventional pilot check valve is applied, in order to prevent the cylinder from dropping down, it is necessary to stop the pressure supply after the valve has been placed in a valve closed state. However, the conventional pilot check valve is not equipped with a function for detecting the closing of the valve. Further, the poppet valve plug and the pilot piston are provided separately from each other, and the position of the pilot piston does not always correspond to the state of the poppet valve plug. Thus, even if the position of the pilot piston is detected, it is difficult to determine whether the poppet valve plug is seated or not, based on the detected position. Therefore, in the conventional pilot check valve, it is impossible to suitably detect whether the valve is in a valve closed state or not.